A novel proteinase, acrolysin, in mammalian testes and spermatozoa cleaves the amino peptide bond of hydrophobic aminoacyl residues. Acrolysin activates proacosin to acrosin, one of the proteinases involved in sperm penetration of ova. The role of acrolysin action and the subsequent molecular events will be evaluated to determine reactions that may be inhibited and thus permit contraception. Combination acrolysin-acrosin inhibitors may serve as contraceptive agents in the male. Acrolysin inhibitor moieties may also serve as adducts to affinity columns for acrolysin purification.